1. Field of the Invention
The present invention relates to a method and apparatus for developing a compressed bandwidth frequency modulation signal format, and more particularly relates to a method and apparatus for increasing the playing time per side of a videodisc by compressing the signal bandwidth of the recorded information preferably by a factor of two. Method and apparatus are described and shown for modulating carrier and subcarrier frequencies with video and audio information and for selecting the carrier and subcarrier frequencies such that the resultant frequency components are arranged throughout the frequency spectrum to optimize the information storage capability of the recording medium with minimal degradation of quality of the reproduced program.
2. Brief Description of the Prior Art
The present invention concerns the signal formatting of a frequency modulated carrier containing composite video information, including chroma information and at least one audio channel of information. Although the invention can be applied to broadcast transmission systems as well, the following description will be concerned with applying the inventive concepts to the recording and playback processes of a videodisc system, and it will be appreciated that such limited scope of description is for the purpose of convenience in describing the development of a signal format and that the invention is equally applicable to line signal transmissive systems, video tape recording systems, and broadcast transmission systems.
For the same reason of convenience as indicated in the previous paragraph, the prior art of concern will relate to frequency modulation signal format processing, and particularly such processing as used in connection with the recording onto a recording medium and the playing back from the recording medium of an audio/video program.
An example of a recording medium with which the present invention has great applicability is the optical disc. An optical disc is a clear plastic disc approximately the size of an LP phonograph record, which has an information carrying surface embedded in its interior beneath a clear exterior surface. Such information carrying surface has a relatively uniform optical property which is altered along a path in a prescribed manner so as to impart informational content to the surface which can then be "read" by way of a beam of light imaged onto the surface. For example, a uniformly reflective surface may be provided and small indicia, such as small pits, having altered reflective properties from that of the remainder of the surface may be arranged sequentially in circular or spiralling tracks. The disc is read by imaging a beam of light onto a track and causing the disc to rotate so that the spot of light scans along the track. The light which is reflected off of the surface is detected by a photodetector which produces an electrical signal in response to the sensing of light. Variations in the intensity of the reflected light due to the alternate presence or absence of the indicia cause variations in the electrical signal. These variations in the electrical signal represent the information stored on the disc.
Information is stored on the disc in the form of variations in the spatial frequency of the indicia in the track, and also in "duty cycle" variations, that is, variations in the relative length of the indicia as compared with the "land" of reflective surface between them. When read by a beam of light and photodetector, as described above, the spatial frequency and duty cycle variations of the indicia cause time/frequency variations and time/duty cycle variations of the detected electrical signal.
U.S. Pat. No. 3,893,163 ("the 163 patent") which was issued July 1, 1975, discloses a method of recording a video signal onto a recording medium, such as a disc, according to which the luminance and chrominance signals are separated from one another prior to recording. The luminance information is then used to frequency modulate a carrier while the chrominance information is used to modulate a subcarrier. The frequency modulated carrier and subcarrier are linearly summed, resulting in "duty cycle" modulation of the carrier by the subcarrier at the zero crossing points of the carrier. That is, the length of the positive half of the signal cycle may be modulated with respect to the length of the negative half of the cycle. The zero crossings of the composite signal are detected and used to generate a square wave of constant amplitude. This square wave signal may be used to generate the indicia on an optical disc such that the luminance information is contained in the spatial frequency variation of the recorded indicia, and the chrominance information is contained in the duty cycle variation of the recorded indicia. Also disclosed in the U.S. Pat. No. 3,893,163 is the technique of summing a frequency modulated sound subcarrier with the modulated chrominance subcarrier and luminance carrier prior to detecting the zero crossings of the composite signal to generate the aforementioned square wave signal.
While the method described in the U.S. Pat. No. 3,893,163 has the advantage of providing a technique for recording color video information and audio information onto an amplitude insensitive recording medium such as an optical disc, it has the disadvantage of requiring that the chrominance information be totally separated from the luminance information in the frequency spectrum.
Another technique, described in an article entitled "System Coding Parameters, Mechanics and Electro-Mechanics of the Reflective Videodisc Player", by P. W. Bogels and N. V. Philips, presented at the IEEE 17th Chicago Spring Conference on Consumer Electronics, June 8, 1976, has been used successfully in commercial applications. This technique, as reported by Bogels, involves frequency modulating a carrier signal with a standard NTSC color video signal, and then modulating the zero crossings of the resultant FM signal with a frequency modulated audio subcarrier in a manner similar to the duty cycle modulation disclosed in the U.S. Pat. No. 3,893,163.
One problem encountered in recording information on an optical disc, using any method, is fitting the information on the discs within the bandwidth limit of the disc. Optical discs made commercially for video recording and playback have an upper cutoff frequency of about 13 mHz at the inner radius. Fitting luminance, chrominance and audio signals onto such a bandwidth limited medium requires care to ensure that sidebands and intermodulation products do not interfere with signal recovery.
A successful format for optical discs which has been implemented with the modulation technique described above according to the Bogels article involves the provision of a video carrier signal frequency modulated with a composite video signal such that a carrier frequency of approximately 8.1 mHz corresponds to blanking level, sync tips correspond to approximately 7.6 mHz, and full white corresponds to approximately 9.3 mHz. Two separate frequency modulated audio subcarriers are positioned in the spectrum at approximately 2.3 mHz and 2.8 mHz. This format is described in some detail in the Bogels article. The format described by Bogels is presently the standard format used in high quality consumer and industrial players and discs of the optical type, and will be referred to hereinafter as a full-luminance/full-color signal format.
For a number of years the full-luminance/full-color signal format has been considered to be the optimum arrangement for recording high quality audio and video information on commercial optical videodiscs. Furthermore, while the aforementioned format yields high quality optical videodisc recordings, an improved method of recording audio and video information onto a recording medium having a limited bandwidth with minimal interfering effect of intermodulation products and improved suppression of second order intermodulation products from an audio subcarrier and video carrier can be found by reference to U.S. patent application Ser. No. 316,015 filed Oct. 28, 1981 and entitled "IMPROVED AUDIO/VIDEO SIGNAL RECORDING METHOD", assigned to the assignee of the present invention.
Another variation of signal format developed specifically for recording on a videodisc can be found in U.S. Pat. No. 3,969,756 to Palmer et al.
The Palmer et al patent relates generally to a disc record structure permitting high density storage of related luminance, chrominance, and multi-channel sound information and to the recording apparatus and methods for recording and playing back such related information. The Palmer et al system was created so as to be able to fit all of the luminance, chrominance, and sound information onto the videodisc structure within the confines of the inherent practical limitations in the recording process as to the highest instantaneous frequency readily recordable on the chosen medium. The medium of concern in the Palmer et al patent is of the variable capacitance form, wherein an information track incorporates geometric variations in the bottom of a spiral groove on a disc, the surface of which comprises conductive material covered with a thin coating of dielectric material. Variations in the capacitance presented between a conductive electrode on a tracking stylus and the conductive material of the disc occur as the disc is rotated by a supporting turntable, and the capacitance variations are sensed to recover the recorded information. However, the relatively high frequency location of the color subcarrier and its sidebands in the NTSC format would, when using the band limited system of Palmer, et al leads to unacceptable playback signal quality. Additionally, serious problems develop with undesirable beats when using the unmodified NTSC format with its high frequency location for color information.
Accordingly, the Palmer et al system was structured to sacrifice picture detail, luminance detail, and some signal-to-noise ratio figures in exchange for a functional and relatively adequate frequency spectrum packing density for the various intelligence information signal elements. As a result, a reasonable color picture with accompanying two-channel audio programming can be recorded and played back with reasonably good fidelity and limit the upper range of the frequency carrier deviation to the still rather high figure of 6.3 mHz (peak white representative).